In the manufacture of MEMs device, the integration of very sensitive moving mechanical parts causes a very serious challenge because:                These very sensitive mechanical parts are typically made of silicon (polysilicon or silicon-germanium);        The sacrificial material underlying these mechanical parts to be released is typically silicon oxide;        The etch-stop layer underlying this silicon oxide sacrificial layer is typically silicon nitride or silicon (polysilicon or silicon-germanium);        The mechanical release of the mechanical parts requires the removal of the sacrificial material in liquid HF-based chemistries;        The surface tension of these liquid HF-based chemistries is high enough to cause stiction of the released mechanical parts onto the underlayers of silicon nitride;        
Liquid HF chemistries are not suitable for stiction-free releases of sensitive MEMS devices. Vapor HF has been used to release such devices without stiction. Unfortunately, vapor HF also attacks the underlying silicon nitride, producing an undesirable fluorinated silicon nitride compound with a rough surface. Fortunately, it has been reported that this fluorinated compound can be evaporated at high temperature using atmospheric pressure ovens operated under a nitrogen or inert ambient and/or at high temperature using vacuum ovens operated under a vacuum as to leave a clean surface under the removed sacrificial layer. Unfortunately, the fluorinated compound present onto the MEMS devices prior to such an ex-situ evaporation is indeed toxic and its manipulation involves the exposure of operators to this toxic fluorinated silicon nitride compound and/or to toxic vapors of this fluorinated silicon nitride compound resulting from its evaporation. More, the fluorinated compound is indeed unstable in presence of moist air and result in non-evaporable residues that cannot be evaporated in the ex-situ vacuum oven
The design of a custom anhydrous HF chamber capable of anhydrous HF release at a high enough temperature and at a good enough vacuum can result in a residue-free release of the micro-devices integrating very sensitive moving mechanical parts. Similarily, such a custom anhydrous HF chamber capable of anhydrous HF release at a high enough temperature but at a vacuum level which is not yet low enough to prevent the formation of the toxic fluorinated silicon nitride residues but which is indeed capable of an in-situ evaporation of the toxic fluorinated silicon nitride residues can result in a safe operation and in a residue-free release of the micro-devices integrating very sensitive moving mechanical parts.
Stiction Issues with Liquid Buffered HF and Non-Buffered HF Solutions
Liquid buffered HF and non-buffered HF solutions have been used to mechanically release the sacrificial oxides underlying the silicon-based (polysilicon-based or silicon-germanium-based) structures, such as the ones shown in FIG. 1. The following references are cited as prior art covering liquid HF release processes:                G. Matamis, B. Gogoib, D. Monkb, A. McNeilb, V. Burrows, “Release etch modeling analysis and the use of Laser Scanning microscopy for etch time prediction of micromachined structures”, Proc. of SPIE Vol. 4174, Micromachining and Microfabrication Process Technology VI, ed. J. Karam, J. Yasaitis (September 2000);        K. R. Williams, R. S. Muller, “Etch Rates for Micromachining Processing”, Journal of Microelectromechanical Systems, Vol. 5, No. 4, December 1996, pp. 256-269;        K. R. Williams, K. Gupta, M. Wasilik, “Etch Rates for Micromachining Processing—Part II”, Journal of Microelectromechanical Systems, Vol. 12, No. 6, December 2003, pp. 761-778;        J. Buhlery, F. P. Steiner, H. Baltes “Silicon Dioxide Sacrificial Layer Etching in Surface Micromachining”, J. Micromech. Microeng. 7 (1997) R1-R13;        
The above references show that HF solutions buffered with 40 wt % ammonium fluoride, NH4F, can be used to remove the sacrificial layer of silicon dioxide, SiO2, because the ammonium fluoride buffer maintains a stable pH and a stable release rate over time. Since ammonium fluoride has a melting point of 993° C. and a boiling point of 1700° C. it is then solid at room temperature. Since ammonium fluoride has a solubility limit of 40 grams/liter of water @ 15° C., it is typically used as a water-based ammonium fluoride solution, NH4F(aq.), at a concentration of 40 wt % NH4F in water. Since a 40 wt % NH4F has a pH of 6.0 and boiling point of 106° C., it is then liquid at room temperature. These buffered liquid HF solutions remove the sacrificial layer of silicon oxide, SiO2, by producing the ammonium fluorosilicate, (NH4)2SiF6, and more water solvent to dissolve the ammonium fluorosilicate by-product:SiO2(s)+4HF(aq.)+2NH4F(aq.)→(NH4)2SiF6(aq.)+2H2O   (I)
The ammonium fluorosilicate (also called ammonium silicofluoride, ammonium fluosilicate, ammonium hexafluorosilicate or bararite) is a solid white cubic (2.011 g/cm3) or triclinic (2.152 g/cm3) crystal at room temperature. It has a high solubility of 250 grams/liter in water @ 20° C.:(NH4)2SiF6(aq.)⇄2NH4+(aq.)+SiF6−2(aq.)
This means that improperly rinsed BHF released wafers will result in an undesirable precipitation of solid ammonium fluorosilicate, (NH4)2SiF6, crystals under the released mechanical parts. This clearly undesirable effect related to the use of BHF solutions is to be prevented in the manufacturing of MEMS devices.
The ammonium fluorosilicate decomposes at 100° C. and results in the formation of volatile ammonia, NH3, of volatile silicon tetrafluoride, SiF4, and of another undesirable white solid by-product, ammonium bifluoride, NH4HF2:(NH4)2SiF6(aq.)→NH4HF2(aq.)+SiF4(g)+NH3(g)
Fortunately, ammonium bifluoride (also called ammonium hydrogendifluoride) has a solubility of 630 grams/liter in water @ 20° C.:NH4HF2(aq.)⇄NH4+(aq.)+HF2−(aq.)
Ammonium bifluoride has a melting point of 125° C. and a decomposition temperature of 238° C.:NH4HF2(aq.)→NH4F(aq.)+HF(aq.)
Because ammonium fluoride, NH4F, has a melting point of 993° C., improperly rinsed mechanical parts released with BHF solutions and heated to high temperatures will also result in undesirable refractory solid residues accumulating under the improperly rinsed mechanical parts. Because advanced MEMS devices incorporating deep cavities and narrow access openings that cannot easily be rinsed, the use of BHF solutions in the manufacturing of such advanced MEMS devices gets much more complicated than the use of non-buffered HF solutions which do NOT result in the formation of such solid by-products.
The cited references also show that the non-buffered 49 wt % HF (in water) solution is used to remove the sacrificial layer of silicon oxide, SiO2. This 49 wt % HF non-buffered solution (without NH4F) has a boiling point of 106° C. and is then liquid at room temperature. The 49 wt % HF liquid solution removes the sacrificial layer of silicon oxide by producing the fluorosilicic acid, H2SiF6, and more water solvent to dissolve the fluorosilicic acid by-product:SiO2(s)+6HF(aq.)→H2SiF6(aq.)+2H2O  (I)
Since the fluorosilicic acid (also called hexafluorosilicic acid, hydrogen hexafluorosilicate, hydrofluorosilicic acid, dihydrogen hexafluorosilicate, hexafluorosilicate (2-) dihydrogen, hydrofluorosilicic acid, hydrogen hexafluorosilicate, hydrogen hexafluorosilic acid, sand acid, silicate (2-) hexafluorodihydrogen, silicic acid, silicofluoric acid, silicofluoride or silicon hexafluoride dihydride) has a melting point of −15.5° C. and a boiling point of 105° C., it is then liquid at room temperature. The fluorosilicic acid, H2SiF6, is very soluble in water:H2SiF6(aq.)⇄2H+(aq.)+SiF6−2(aq.)
FIG. 2 shows the chemical structure of the formed fluorosilicic acid liquid by-products in the solution.
As shown in FIG. 3, non-buffered liquid 49 wt % HF solution can very efficiently release rigid mechanical structures without any solid residues. In this FIG. 3, a machined device has been released by the removal of the local underlying oxide layer.
Unfortunately, the surface tension of liquid 49 wt % HF solution causes a very undesirable stiction effect on more flexible and more fragile mechanical parts than such rigid silicon blocks. FIG. 4 shows various 3D interferometric images of such an undesirable stiction effect on long and fragile cantilevers. Since advanced MEMS devices incorporate many such fragile mechanical parts prone to stiction it is necessary to eliminate this undesirable effect related to the use of liquid HF solutions.
The Physics of the Undesirable Stiction Effect Observed with Liquid HF Solutions
FIG. 5 shows the shape of a liquid droplet deposited onto the surface of a given solid. From a thermodynamic standpoint, the work of adhesion of the interface is the amount of energy required to create free surfaces from the bonded materials:WA=γLA+γSA−γLS where γLA and γSA are the specific surface energies of the liquid and the solid in ambient air respectively and where γLS is the interfacial energy between the solid and the liquid. The work of adhesion is often determined by contact angle measurements using a goniometer. If the tested liquid droplet is in thermodynamic equilibrium with the solid, then:γSA=γLS+γLA cos Θwhere Θ is the contact angle between the liquid droplet and the solid surface. The work of adhesion now can be expressed with the Young-Dupré equation:WA=γLA+γLS+γLA cos Θ−γLS=γLA(1+cos Θ)
The work of adhesion depends on the liquid surface tension (γLA) and the contact angle (Θ). The following two cited references:                C. H. Mastrangelo, “Mechanical Stability and Adhesion of Microstructures Under Capillary Forces—Part I: Basic Theory”, Journal of Microelectromechanical Systems, Vol. 2, No. 1, March 1993, pp. 33-43;        C. H. Mastrangelo, “Mechanical Stability and Adhesion of Microstructures Under Capillary Forces—Part II: Experiments”, Journal of Microelectromechanical Systems, Vol. 2, No. 1, March 1993, pp. 44-55;show that a cantilever beam shorter than a certain characteristic length called the detachment length, Ld, is stiff enough to free itself completely from the underlying material. In contrast, a cantilever beam longer than this detachment length, Ld, will not be stiff enough and will be pulled-down by the force generated by the surface tension of the liquid used for its mechanical release and will be stuck to the underlying material after the evaporation of this liquid. Using the nomenclature presented in FIG. 4, the following equation describes the relationship between the detachment length, the cantilever dimensions (t=thickness, l=length, w=width, h=height of sacrificial material), the cantilever's material properties (E=Young modulus), the liquid surface tension (γLA) and the measured contact angle (Θ):Ld=[2E/(9γLA cos Θ)]1/4[h2t3/(1+t/w)]1/4 i.e. the detachment length, Ld, increases linearly as function of the cantilever's design characteristics [h2t3/(1+t/w)]1/4 with a slope [2E/(9γLA cos Θ)]1/4 dictated by the Young modulus of the cantilever, the liquid surface tension (γLA) and the measured contact angle (Θ). It is then predicted that the detachment length (Ld) of a cantilever (t, l, w) made of a material of a Young Modulus, E, mechanically released over a sacrificial layer (h) will be shorter when mechanically released by a liquid having a larger surface tension (γLA) and larger contact angle (Θ).        
The 3D interfometric image of FIG. 4 shows a very clear demonstration of the relatively short detachment length (Ld) of cantilevers released in a non-buffered 49 wt % HF solution.
The chemicals used to remove the sacrificial oxide layer over the underlying surface of silicon, silicon nitride or other non-removed layer dictate the behavior of the various exposed surfaces in liquid water i.e. dictates if these become hydrophilic or hydrophobic surfaces.
FIG. 6 is a reprint of the upper-cited C. H. Mastrangelo reference. It shows the linear relationship between the measured detachment length, Ld, of cantilevers and the cantilever's design characteristics [h2t3/(1+t/w)]1/4 for hydrophobic (water rinse after mechanical release in HF solutions) and hydrophilic (water rinse after sulphuric acid & hydrogen peroxide exposure after mechanical release in HF solutions) cantilevers. This FIG. 6 shows that there is basically very little difference between measured detachment length, Ld, of hydrophilic and hydrophobic cantilevers.
When the mechanical release of the mechanical parts is done in a non-buffered 49 wt % HF solution, a final rinse in de-ionized water is performed to stop the release and to eliminate the residual H+ and F− ions from the exposed surfaces. The following reference:                D. R. Ride, “Handbook of Chemistry and Physics”, 71th Edition, 1991, CRC Press, Boca Raton, Fla., USA;shows that de-ionized water has a surface tension of 73.05 mN/m at a temperature of 20° C. The high surface tension is limitative to achieve the long detachment length, Ld, required for fragile mechanical parts composing advanced MEMS devices. Other chemicals than de-ionized water having lower surface tension, γLA, should be used to achieve better resistance to stiction (i.e. lower work of adhesion, WA, and longer detachment length, Ld).Stiction Reduction using Lower Surface Tension Liquids        
Lower surface tension liquids have been used to try minimizing the stiction effect. The following reference:                O. Raccurt, F. Tardif, F. Arnaud d'Avitaya, T. Vareine, “Influence of liquid surface tension on stiction of SOI MEMS”, Journal of Micromechanics and Microengineering, Vol. 14, (2004) 1083-1090is used as prior art reference to demonstrate that lower surface tension liquids slightly help minimizing the undesirable stiction effect. It compares the detachment length, Ld, of various silicon cantilevers released from their underlying oxide using either:        De-ionized water, having a high surface tension, γLA, of 73.1×10−3 N/m;        Isopropyl alcohol, having a mid surface tension, γLA, of 21.7×10−3 N/m;        Pentane, having a very low surface tension, γLA, of 13.7×10−3 N/m;        
In order to verify the effect of the surface tension, γLA, on the detachment length, Ld:Ld=[2E/(9γLA cos Θ)]1/4[h2t3/(1+t/w)]1/4 four types of SOI substrates have been used to machine cantilevers of various length:                SOI substrate #1: top silicon thickness, t, of 20.0 μm over 0.5 μm sacrificial oxide, h;        SOI substrate #2: top silicon thickness, t, of 15.0 μm over 2.0 μm sacrificial oxide, h;        SOI substrate #3: top silicon thickness, t, of 20.0 μm over 2.0 μm sacrificial oxide, h;        SOI substrate #4: top silicon thickness, t, of 20.0 μm over 3.0 μm sacrificial oxide, h;        
This reference shows that the various cantilevers are machined in the top silicon region (Young modulus of 170 GPa) as to reach the bottom sacrificial oxide. Following proper resist strip, the cantilevers are released from their underlying oxide using a non-buffered 49 wt % HF solution followed by a rapid displacement of the 49 wt % HF releasing solution using either:                De-ionized water;        Isopropyl alcohol;        Pentane;        
FIG. 7 is a reprint of the upper cited O. Raccurt reference. This FIG. 7 shows the set-up used by this cited reference to ensure this rapid replacement of the non-buffered 49 wt % HF solution by de-ionized water and then the replacement of this de-ionized water by either one of:                De-ionized water;        Isopropyl alcohol;        Pentane.        
This FIG. 7 shows that the steps a) to h) of releasing, rinsing and drying the various cantilevers machined in the four types of SOI wafers are performed in the same bath without liquid/gas interface:                a) & b) Releasing of the cantilevers by etching the sacrificial layer in 49 wt % HF;        c) Stop etching the sacrificial oxide using a “piston” flow of de-ionized water removing the 49 wt % HF by overflow;        d) Rinsing using recirculation;        e) Rinse using a “piston” flow of de-ionized water removing the recirculated de-ionized water by overflow;        f) Introduction of either isopropyl alcohol or pentane onto the de-ionized water;        g) SOI wafer drying by displacement through the layer of either isopropyl alcohol or pentane;        h) Recovery of the drying liquid by overflow.        
FIG. 8 is also reprint of the cited O. Raccurt reference. This FIG. 8 shows the observed effect of the surface tension of the drying liquid on the detachment length, Ld, of the various cantilevers machined in the four types of SOI substrates. It appears that there is not that much differences between the detachment length, Ld, and the surface tension of the drying liquid for three out of four types of SOI substrates. For the fourth type of SOI substrate, there is an increase of the detachment length, Ld, using lower surface tension pentane as replacement of de-ionized water.
Such small differences of detachment length, Ld, of sensitive mechanical parts using smaller surface tension drying liquids do not fulfill the requirements for most advanced MEMS devices integrating a large number of sensitive mechanical parts and does not safely prevent the stiction issues associated with such type of devices.
Another technique is to be used to prevent stiction of such advanced MEMS devices.
Stiction Reduction using Super-Critical CO2 Drying
Super-critical CO2 drying has been used to try minimizing the stiction effect. The following references:                J. J. Sniegowski, C. Smith, “An Application of Mechanical Leverage to Microactuation”, 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Proc., Transdiucers '95 Eurosensors IX, Stockholm, Sweden, Vol. 2, Jun. 25-29, 1995, pp. 364-367;        J. W. Judy, R. S. Muller, “Batch-Fabricated, Addressable, Magnetically Actuated Microstructures”, Technical Digest, Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, Jun. 3-6, 1996;        C. W. Dyck, J. H. Smith, S. L. Miller, E. M. Russick, C. L. J. Adkins, “Supercritical carbon dioxide solvent extraction from surface-micromachined micromechanical structures”, SPIE Micromachining and Microfabrication, October, 1996;        C. H. Mastrangelo, “Adhesion-related failure mechanisms in micromechanical device” Tribology Letters, Vol. 3, No. 3, June 1997, pp. 223-238;        C. J. Kim, J. Y. Kim, B. Sridharan, “Comparative Evaluation of Drying Techniques for Surface Micromachining”, Sensors and Actuators, A 64, 1998, pp. 17-26;        L. Lin, R. T. Howe, A. P. Pisano, “Microelectromechanical Filters for Signal Processing”, Journal of Micromechanical Systems, Vol. 7, No. 3, September 1998, pp. 286-294;        J. Zou, C. Liu, J. Chen, S. M. Kang, “Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication Systems,” IEEE IEDM, 2000, pp. 403-406;        L. Lin, “MEMS Post-Packaging by Localized Heating and Bonding”, IEEE Transactions on Advanced Packaging, Vol. 23, No. 4, November 2000, pp. 608-616;        N. Hoivik, Y. C. Lee, V. M. Bright, “Flip Chip Variable High-Q MEMS Capacitor for RF Applications, Proceedings of IPACK'01, The Pacific RIM/ASME International Electronic Packaging Technical Conference and Exhibition, Kauai, Hi., USA, Jul. 8-13, 2001;        W. I. Jang, C. A. Choi, M. L. Lee, C. H. Jun, Y. T. Kim, “Fabrication of MEMS devices by using anhydrous HF gas-phase etching with alcoholic vapor”, Journal of Micromechanics and Microengineering, Vol. 12, 2002, pp. 297-306;are used as cited prior art to demonstrate that the supercritical CO2 drying helps minimizing the undesirable stiction effect. The supercritical drying technique uses CO2 as the supercritical fluid because of its relatively low supercritical pressure (1073 psi) and temperature (31.1° C.). The steps involved in the supercritical CO2 drying of released micro-structures are:        Release by immersion in liquid HF solutions;        Surface passivation of the released silicon structure by immersion in a sulfuric peroxide or in a hydrogen peroxide solution, thus resulting in hydrophilic silicon surfaces;        Thorough de-ionized water rinses followed by a solvent or a alcohol soak to displace the water;        Placement of the solvent- or alcohol-soaked samples in a supercritical drying chamber for drying. Once in the pressure vessel, the solvent or alcohol is completely displaced by liquid CO2 at a pressure of about 1200 psi, then at a pressure higher than the supercritical pressure of 1073 psi.        FIG. 9 shows the mechanism behind the super-critical CO2 drying. Drying takes place by passing from the liquid phase to the gas phase through the supercritical region and above the supercritical point (31.1° C., 1073 psi):        First, the CO2-pressurized vessel is heated until the liquid CO2 makes the transition to the supercritical phase (T>31.3° C.);        Venting the vessel to rapidly reduce the pressure isothermally above the CO2 supercritical temperature results in dried “stiction-free” surfaces.        
Because the liquid-to-vapor transition occurs in the supercritical region, there are no attractive capillary forces to cause stiction during the drying phase.
This supercritical CO2 drying technique is not that well suited for mass production because the high pressure of about 1200 psi (required to dry the released structures by passing from the liquid phase to the gas phase through the supercritical region and above the supercritical point of 31.1° C. and 1073 psi) imposes the use of bulky and thick walls mechanical chambers to manually load and dry, one-by-one, the solvent-soaking substrates integrating advanced MEMS devices having their sensitive mechanical parts already released and yet dripping solvent. The most advanced supercritical CO2 dryer currently available is the “Automegasamdri®-915 Series C” from Tousamis:                http://tousimis.com/Specs/8785D.pdf        
FIG. 10 shows the picture of this system and clearly demonstrates the automation and mass production limitations of this supercritical CO2 drying technique which imposes the manually loading, one-by-one, of yet solvent-soaking substrates.
Incompatibility of Standard CMOS Metals to Liquid HF
The above-mentioned release techniques involving liquid HF suffer from either stiction complications or lack of automation, thus limiting the mass production of advanced MEMS devices integrating sensitive mechanical parts.
Over these very serious problems, liquid-based HF solutions also suffer from metal incompatibility issues. These incompatibility issues are due to the fact that advanced MEMS devices typically integrate digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions. Since this CMOS electronics needs to be exposed to the chemicals used for the removal of the oxide sacrificial material to allow the release the mechanical parts of these advanced MEMS devices, the liquid-based HF solutions should not chemically attack the metal-based interconnections of this CMOS electronics. Unfortunately almost all CMOS interconnect materials, namely aluminum alloys (such as aluminum-silicon, aluminum-copper or aluminum-silicon-copper alloys), titanium (and titanium compounds such as titanium nitride) and copper are rapidly attacked by these liquid-based HF solutions. The following documents are used as prior art references to demonstrate these undesirable limitations:                K. R. Williams, R. S. Muller, “Etch Rate for Micromachining Processing”, Journal of Micromechanical Systems, Vol. 5, No. 4, December 1996, pp. 256-294;        A. Witvrouw, B. Du Bois, P. De Moor, A. Verbist, C. Van Hoof, H. Bender, K. Baert, “A comparison between wet HF etching and vapor HF etching for sacrificial oxide removal”, Proc. SPIE Micromachining and Microfabrication Process Technology VI; September, 2000, Vol. 4174, 2000, pp. 130-141;        K. R. Williams, K. Gupta, “Etch Rate for Micromachining Processing—Part II”, Journal of Micromechanical Systems, Vol. 12, No. 6, December 2003, pp. 761-778;        
This clearly limits the use of liquid-based HF solutions for the mass production of advanced MEMS devices integrating digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions.